The subject invention relates to the extrusion of molten thermoplastic film onto a quench roll casting drum.
It is well known to produce cast film from certain thermoplastic polymers, e.g., polyethylene terephthalate (PET), which film is useful for many applications in packaging, electronics, etc. In methods for the production of such films, molten polymer is usually extruded onto the surface of a quench roll and subsequently oriented by known stretching techniques. In such methods where the film is extruded at temperatures above or near the polymer melting point, it is important to quickly cool the freshly formed film to a temperature below the second order transition temperature, i.e., that temperature at which, as temperature decreases, the polymer goes from a flexible state to a more rigid, glassy state, in order to prevent crystallization which would interfere with subsequent orientation of the film. It is, therefore, important to achieve quick, intimate contact between the extruded film and the quench roll, (referred to as "pinning") which includes preventing excess air from being trapped between the film and the roll.
In Heyer, U.S. Pat. No. 4,310,295 (Heyer 295), a vacuum pinning device is disclosed for the uniform pinning of molten web material (especially PET) onto a quench roll by means of at least two vacuum zones interacting through a baffle. The disclosure of Heyer 295 is hereby incorporated by reference.
In Heyer, U.S. Pat. No. 4,501,712 (Heyer 712), a process is disclosed for using a vacuum pinning web casting device, such as disclosed in Heyer 295, wherein the quality of a film such as PET is improved by creating a surface roughness on the quench roll of at least 3 microinches (0.08 micrometers) while simultaneously applying a vacuum force to the line of contact between the web and the roll thereby preventing excess air from being entrapped between the film and the roll. The disclosure of Heyer 712 is also incorporated herein.
As indicated in Heyer 712, a problem with using vacuum assisted film pinning devices is the avoidance of both "mottle" and "bursting". Mottle manifests itself when there is incomplete contact of the film with the quench roll surface causing a dimpled surface resembling an orange peel. Mottle is the result of too little vacuum pinning force. Bursting results when too high a vacuum force is used; the film may be drawn inward toward the vacuum source and away from the quench roll surface. The difference between these two pressures is the operating range or "window". It is advantageous from an operating standpoint to have as large a window as possible.
The process in Heyer 712 in using a roughened quench roll surface enabled one to obtain a larger operating window than a similar process using a highly polished quench roll. Unfortunately, when using the process to cast films significantly less than 5 mils (127 micrometers) in cast thickness, especially of about 3.5 mils (90 micrometers) and below, it becomes difficult if not impossible to maintain a suitably wide operating window.